Activist investor Carl Icahn is mulling steps to shake up the board of SandRidge Energy after the oil and gas company adopted a poison pill meant to prevent him from scooping up more shares.

The octogenarian billionaire became the company's largest shareholder after disclosing a 13.5% stake in SandRidge last week.

He has taken issue with the company's plan to buy Bonanza Creek Energy, calling it "overpriced" and a "mistake" that would destroy shareholder value. SandRidge announced earlier this month it would buy Bonanza for $750 million in cash and stock.

SandRidge instated the so-called poison pill in an effort to stop investors from buying more than a 10% stake in the company and to deter existing shareholders from adding to their current positions.

"You adopted a poison pill that is a complete travesty and represents a new low in corporate governance," fumed Icahn in an open letter to the company on Thursday. "Such actions would make a totalitarian dictator blush, yet you take them with a straight face."

In the letter, Icahn asked for clarity on the measure and whether he can still discuss his opposition to the Bonzanza Creek deal with other shareholders or organize efforts to block the vote without triggering the poison pill.

He said he is also considering picking a board fight and calling on shareholders to vote to remove and replace current members of the board.

Icahn has made a name for himself as a no-holds-barred shareholder activist who takes a large stake in a company and uses it to aggressively agitate for change. Over the last several decades, he's picked fights with companies ranging from Time Warner and eBay to Texaco and U.S. Steel.

Icahn is joined by Fir Tree Partners, another investor in SandRidge, in its opposition of the tie-up with Bonanza. Fir Tree has complained that the deal is not consistent with the strategy outlined in the company's 2016 exit from bankruptcy and that it's paying too much for a business that doesn't make strategic sense.

"[It] reminds us of SandRidge's prior history when this same management team acquired disparate assets and added leverage with reckless abandon," wrote managing directors Evan Lederman and David Proman in an open letter. Fir Tree owns about 8% of the company.

SandRidge described the measure as a way to preserve the ability of its shareholders to consider the Bonanza deal and vote as they wish. In a statement, the company said that its "independent board thoroughly evaluated and unanimously supports the Bonanza Creek transaction" and its forthcoming proxy materials "will give shareholders the ability to fully review the merits of the transaction."